cc. Graphideae. Several members of the Graphidaceae were studied by G. Wolff[600]: she demonstrated the presence of carpogonia with emerging trichogynes in Graphis elegans, a species which is distinguished by the deeply furrowed margins of the lirellae ([Fig. 89]). Before the carpogonia appeared it was possible to distinguish the cushion-like primordial tissue of the apothecium in the thallus which is almost wholly immersed in the periderm layers of the bark on which it grows. The trichogynes were very sparingly septate, and a rather large nucleus occupied a position near the tip of the terminal cell. The dark carbonaceous outer wall makes its appearance in this species at an early stage of development along the sides of the lirellae, but never below, as there is always a layer of living cells at the base. After the first-formed hymenium is exhausted, these basal cells develop a new apothecium with a new carbonaceous wall that pushes back the first-formed, leaving a cleft between the old and the new. This regenerating process, somewhat analogous to the formation of new apothecia in Pertusaria, may be repeated in Graphis elegans as many as five times, the traces of the older discs being clearly seen in the channelled margins of the lirellae.
Fig. 98. Cladonia decorticata Spreng. Vertical section of squamule and primordium of podetium. a, developing podetium; b, probably fertile hyphae; c, cortical tissue; d, gonidial cells. 1 × 600 (after Krabbe).
dd. Cladoniae. The chief points of interest in the Cladoniae are the position of the apothecial primordia and the function of the podetium, which are discussed later[601]. Krabbe[602] determined not only the endogenous origin of the podetium but also the appearance of fertile cells in the primordium ([Fig. 98]). Both frequently take rise where a crack occurs in the cortex of the primary squamule, the cells of the gonidial tissue being especially active at these somewhat exposed places. The fertile hyphae elongate and branch within the stalk of the developing podetium, sometimes very early, or not until there is a pause in growth, when carpogonia are formed. As a rule trichogynes emerge in great numbers[600], generally close to, or rather below, the spermogonia. In Cl. pyxidata[603] the carpogonia are characterized by the large diameter of the cells—three to five times that of the vegetative hyphae. Though most of the trichogynes disappear at an early stage, some of them may persist for a considerable period. As development proceeds, the vegetative hyphae interspersed among the ascogonial cells grow upwards, slender branches push up between them and gradually a compact sheath of paraphyses is built up. The ascogenous hyphae meanwhile spread radially at the base of the paraphyses and the asci begin to form. The apothecia may be further enlarged by intercalary growth, and this vigorous development of vegetative tissue immediately underneath raises the whole fruit structure well above the surface level.
Sättler[604] in his paper on Cladoniae[605] cites as an argument in favour of fertilization the relative positions of carpogonia and spermogonia on the podetia. The carpogonia with their emerging trichogynes being situated rather below the spermogonia. Both organs, he states, have been demonstrated in eleven species; he himself observed them in the primordial podetia of Cladonia botrytes and of Cl. Floerkeana.
2. PYRENOLICHENS
a. Development of the Perithecium. It is to Fuisting[606] that we owe the first account of development in the lichen perithecium. Though he failed to see the earlier stages (in Verrucaria Dufourii), he recognized the primordial complex of hyphae in the gonidial zone of the thallus, from which originated a vertical strand of hyphae destined to form the tubular neck of the perithecium. Growth in the lower part is in abeyance for a time, and it is only after the neck is formed, and the fruiting body is widened by the ingrowth of external hyphae, that the asci begin to branch up from the tissue at the base.
Fig. 99. Dermatocarpon miniatum Th. Fr. Vertical section of thallus and carpogonial group × 600 (after Baur).
b. Formation of Carpogonia. Stahl[607] had indicated that not only in gymnocarpous but also in angiocarpous lichens, it would be found that carpogonia were formed as in Collema. Baur[608] justified this surmise, and demonstrated the presence of ascogonia in groups of three to eight, with trichogynes that reached the surface in Endocarpon (Dermatocarpon) miniatum ([Fig. 99]). It is one of the few foliaceous Pyrenolichens, and the leathery thallus is attached to the substratum by a central point, thus allowing in the thallus not only peripheral but also intercalary growth, the latter specially active round the point of basal attachment; carpogonia may be found in any region where the tissue is newly formed, and at any season. The upper cortex is composed of short-celled thick-walled hyphae, with branching vertical to the surface, and so closely packed that there is an appearance of plectenchyma; the medullary hyphae are also thick-walled but with longer cells. The carpogonia of this species arise as a branch from the vegetative hyphae and are without special covering hyphae, so frequent a feature in other lichens. The trichogynes bore their way through the compact cortex and rise well above the surface. After they have disappeared—presumably after fertilization—the vegetative hyphae round and between the ascogonia become active and travel upwards slightly converging to a central point. The asci begin to grow out from the ascogenous hyphae of the base before the vertical filaments have quite pierced the cortex.